This invention relates to a method and a device for processing an STS-1 signal (namely, a synchronous transport signal level 1) into a DS-3 signal (that is, a digital signal level 3). This invention relates also to another method for processing a VT (virtual tributary) into a DS-1 signal (namely, a digital signal level 1).
The STS-1 signal and the VT are prescribed in American National Standard for Telecommunications-Digital Hierarchy-Optical Interface Rates and Specifications (SONET), T1X1.5/90-025R3/T1X1/90-055R2, Draft-November 1990, pages 9-23, published by American National Standards Institute, Inc.
As prescribed in the above-mentioned Standard on Paragraph 5, pages 14-15, the STS-1 signal has a bit rate of 51.840 Mbits/s. The STS-1 signal comprises an STS synchronous payload envelope (STS-1 SPE) comprising first through M-th subframes, where M represents a plural natural number. In a practical use, M is equal to 9. Each of the first through the M-th subframes comprising an STS path overhead (STS-1 POH) and a payload comprising an information part.
As also prescribed in the above-mentioned Standard on page 21, a predetermined one of the STS path overheads of the first through M-th subframes is allocated as an STS path signal label part to indicate whether or not the STS synchronous payload envelope is unequipped. The STS path signal label part indicates an unequipped code consisting of all zero bits when the STS synchronous payload envelope is unequipped. In the practical use, the STS path overhead of a third subframe is allocated as the STS path signal label part.
The information parts of the payloads of the first through the M-th subframes are used in transporting the DS-3 signal when the STS synchronous payload envelope is not unequipped. When the STS synchronous payload envelope is not unequipped, the STS-1 signal is practically used. When the STS synchronous payload envelope is unequipped, the STS-1 signal is not used and is therefore referred to as an unused STS-1 signal. The DS-3 signal has a bit rate of 44.736 Mbits/s. At any rate, the DS-3 signal can be transported by the STS synchronous payload envelope.
Inasmuch as the DS-1 signal has a lower bit rate of 1.544 Mbits/s relative to the DS-3 signal, the DS-1 signal is transferred by the VT of size x defined in the STS synchronous payload envelope as prescribed in the above-mentioned Standard on the left-hand column of page 15. For example, the VT of size 1.5 has a bit rate of 1.728 Mbits/s as prescribed in the above-mentioned Standard on Paragraph 8.1.3 of page 16.
The VT of size x comprises a VT synchronous payload envelope (VT SPE). The VT synchronous payload envelope comprises a VT path overhead (VT POH) and a payload comprising an information part. A part of the VT path overhead is allocated as a VT path signal label part to indicate whether or not the VT synchronous payload envelope is unequipped. The VT path signal label part indicates an unequipped code consisting of all zero bits when the VT synchronous payload envelope is unequipped. The information part of the payload of the VT synchronous payload envelope is used in transporting the DS-1 signal when the VT synchronous payload envelope is not unequipped. When the VT synchronous payload envelope is not unequipped, the VT of size x is practically used. When the VT synchronous payload envelope is unequipped, the VT in question is not used and is therefore called an unused VT.
As mentioned above, the above-mentioned Standard prescribes that the unequipped code should be set in the STS path signal label part for the unused STS-1 signal and that the unequipped code should be set in the VT path signal label part for the unused VT of size x. However, inasmuch as the above-mentioned Standard never prescribes as regards both the information parts of the payloads of the first through the M-th subframes of the STS synchronous payload envelope for the unused STS-1 signal and the information part of the payload of the VT synchronous payload envelope for the unused VT of size x, an unprescribed code consisting of, for example, all zero bits is inevitably transported by the information parts of the unused STS-1 signal and by the information part of the unused VT of size x.
When the unused STS-1 signal and the unused VT of size x are processed in conventional processing devices, every one of the conventional processing devices produces or outputs the unprescribed code instead of the DS-3 and the DS-1 signals. A subsequent terminal equipment unavoidably generates an unnecessary alarm in response to the unprescribed code.